Java – Set BufferedImage alpha mask in Java


I have two BufferedImages I loaded in from pngs. The first contains an image, the second an alpha mask for the image.

I want to create a combined image from the two, by applying the alpha mask. My google-fu fails me.

I know how to load/save the images, I just need the bit where I go from two BufferedImages to one BufferedImage with the right alpha channel.

Best Answer

I'm too late with this answer, but maybe it is of use for someone anyway. This is a simpler and more efficient version of Michael Myers' method:

public void applyGrayscaleMaskToAlpha(BufferedImage image, BufferedImage mask)
    int width = image.getWidth();
    int height = image.getHeight();

    int[] imagePixels = image.getRGB(0, 0, width, height, null, 0, width);
    int[] maskPixels = mask.getRGB(0, 0, width, height, null, 0, width);

    for (int i = 0; i < imagePixels.length; i++)
        int color = imagePixels[i] & 0x00ffffff; // Mask preexisting alpha
        int alpha = maskPixels[i] << 24; // Shift blue to alpha
        imagePixels[i] = color | alpha;

    image.setRGB(0, 0, width, height, imagePixels, 0, width);

It reads all the pixels into an array at the beginning, thus requiring only one for-loop. Also, it directly shifts the blue byte to the alpha (of the mask color), instead of first masking the red byte and then shifting it.

Like the other methods, it assumes both images have the same dimensions.